MathPad is a general purpose graphing scientific calculator for the Macintosh. It uses text worksheets rather than simulating buttons on a hand held calculator. This live scratchpad interface allows you to see and edit your entire calculation. Formulas can be entered directly and different values can be plugged in for easy "what if" calculating.

This little app is really good for any (numerical) tasks you can't be bothered to program in C or are not worth buying MatLab/Mathematica/etc. It can do simple computations, function definitions, arrays, matrices, nested loops, plot in 2D, 3D (either the traditional way or by creating a color map of the values). It's pretty fast.

And it has sliders for variable parameters: if you define a slider, you get a little window with a scroll bar, and as soon as you move the it, the new value is used and everything is recomputed. So if you've got a plot depending on a couple of parameters, you can easily experiment with different values.